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For other meanings of "bridge" see bridge (disambiguation)

A bridge is a structure to cross an open space or gap. Bridges are mostly useful for crossing rivers, valleys, or roads by vehicles but people have also used bridges for a long time for walking. Bridges called highway overpasses are used to avoid traffic control.

Bridges also have uses in the military. Military bridges have to be portable, so that they may be easily moved to the front lines, and so they are much more complex than regular civilian bridges.

The simplest type of a bridge is stepping stones, so this may have been one of the earliest types. Neolithic people also built a form of boardwalk across marshes; examples of such bridges include the Sweet Track and the Post Track in England, approximately 6000 years old. Undoubtedly, ancient people would also have used log bridges; that is a timber bridge that fall naturally or are intentionally felled or placed across streams. Some of the first man-made bridges with significant span were probably intentionally felled trees.

Among the oldest timber bridges is the Holzbrücke Rapperswil-Hurden crossing upper Lake Zürich in Switzerland; the prehistoric timber piles discovered to the west of the Seedamm date back to 1523 BC. The first wooden footbridge led across Lake Zürich, followed by several reconstructions at least until the late 2nd century AD, when the Roman Empire built a 6-metre-wide (20 ft) wooden bridge. Between 1358 and 1360, Rudolf IV, Duke of Austria, built a 'new' wooden bridge across the lake that has been used to 1878 - measuring approximately 1,450 metres (4,760 ft) in length and 4 metres (13 ft) wide. On April 6, 2001, the reconstructed wooden footbridge was opened, being the longest wooden bridge in Switzerland.

The Arkadiko Bridge is one of four Mycenaean corbel arch bridges part of a former network of roads, designed to accommodate chariots, between the fort of Tiryns and town of Epidauros in the Peloponnese, in southern Greece. Dating to the Greek Bronze Age (13th century BC), it is one of the oldest arch bridges still in existence and use. Several intact arched stone bridges from the Hellenistic era can be found in the Peloponnese.

The greatest bridge builders of antiquity were the ancient Romans. The Romans built arch bridges and aqueducts that could stand in conditions that would damage or destroy earlier designs. Some stand today. An example is the Alcántara Bridge, built over the river Tagus, in Spain. The Romans also used cement, which reduced the variation of strength found in natural stone. One type of cement, called pozzolana, consisted of water, lime, sand, and volcanic rock. Brick and mortar bridges were built after the Roman era, as the technology for cement was lost (then later rediscovered).

In India, the Arthashastra treatise by Kautilya mentions the construction of dams and bridges. A Mauryan bridge near Girnar was surveyed by James Princep. The bridge was swept away during a flood, and later repaired by Puspagupta, the chief architect of emperor Chandragupta I. The use of stronger bridges using plaited bamboo and iron chain was visible in India by about the 4th century. A number of bridges, both for military and commercial purposes, were constructed by the Mughal administration in India.

Although large Chinese bridges of wooden construction existed at the time of the Warring States period, the oldest surviving stone bridge in China is the Zhaozhou Bridge, built from 595 to 605 AD during the Sui dynasty. This bridge is also historically significant as it is the world's oldest open-spandrel stone segmental arch bridge. European segmental arch bridges date back to at least the Alconétar Bridge (approximately 2nd century AD), while the enormous Roman era Trajan's Bridge (105 AD) featured open-spandrel segmental arches in wooden construction.

Rope bridges, a simple type of suspension bridge, were used by the Inca civilization in the Andes mountains of South America, just prior to European colonization in the 16th century.

During the 18th century there were many innovations in the design of timber bridges by Hans Ulrich Grubenmann, Johannes Grubenmann, and others. The first book on bridge engineering was written by Hubert Gautier in 1716.

A major breakthrough in bridge technology came with the erection of the Iron Bridge in Shropshire, England in 1779. It used cast iron for the first time as arches to cross the river Severn. With the Industrial Revolution in the 19th century, truss systems of wrought iron were developed for larger bridges, but iron does not have the tensile strength to support large loads. With the advent of steel, which has a high tensile strength, much larger bridges were built, many using the ideas of Gustave Eiffel.

In Canada and the United States, numerous timber covered bridges were built in the late 1700s to the late 1800s, reminiscent of earlier designs in Germany and Switzerland. Some covered bridges were also built in Asia. In later years, some were partly made of stone or metal but the trusses were usually still made of wood; in the United States, there were three styles of trusses, the Queen Post, the Burr Arch and the Town Lattice. Hundreds of these structures still stand in North America. They were brought to the attention of the general public in the 1990s by the novel, movie, and play The Bridges of Madison County.

In 1927 welding pioneer Stefan Bryła designed the first welded road bridge in the world, the Maurzyce Bridge which was later built across the river Słudwia at Maurzyce near Łowicz, Poland in 1929. In 1995, the American Welding Society presented the Historic Welded Structure Award for the bridge to Poland.

Types of bridges

Bridges can be categorized in several different ways. Common categories include the type of structural elements used, by what they carry, whether they are fixed or movable, and by the materials used.

Structure types

Bridges may be classified by how the actions of tension, compression, bending, torsion and shear are distributed through their structure.

Beam bridges are horizontal beams supported at each end by substructure units and can be either simply supported when the beams only connect across a single span, or continuous when the beams are connected across two or more spans. When there are multiple spans, the intermediate supports are known as piers.

A truss bridge is a bridge whose load-bearing superstructure is composed of a truss. This truss is a structure of connected elements forming triangular units.

Cantilever bridges are built using cantilevers -horizontal beams supported on only one end. Most cantilever bridges use a pair of continuous spans that extend from opposite sides of the supporting piers to meet at the center of the obstacle the bridge crosses.

Arch bridges have abutments at each end. The weight of the bridge is thrust into the abutments at either side. The earliest known arch bridges were built by the Greeks, and include the Arkadiko Bridge.

Tied-arch bridges have an arch-shaped superstructure, but differ from conventional arch bridges. Instead of transferring the weight of the bridge and traffic loads into thrust forces into the abutments, the ends of the arches are restrained by tension in the bottom chord of the structure. They are also called bowstring arches.

Suspension bridges are suspended from cables. The earliest suspension bridges were made of ropes or vines covered with pieces of bamboo. In modern bridges, the cables hang from towers that are attached to caissons or cofferdams. The caissons or cofferdams are implanted deep into the bed of the lake, river or sea.

Cable-stayed bridges, like suspension bridges, are held up by cables. However, in a cable-stayed bridge, less cable is required and the towers holding the cables are proportionately higher. The first known cable-stayed bridge was designed in 1784 by C. T. (or C. J.) Löscher.

Fixed or movable bridges

Most bridges are fixed bridges, meaning they have no moving parts and stay in one place until they fail or are demolished. Temporary bridges, such as Bailey bridges, are designed to be assembled, and taken apart, transported to a different site, and re-used. They are important in military engineering, and are also used to carry traffic while an old bridge is being rebuilt. Movable bridges are designed to move out of the way of boats or other kinds of traffic, which would otherwise be too tall to fit. These are generally electrically powered.

Double-decked bridges

Double-decked (or double-decker) bridges have two levels, such as the George Washington Bridge, connecting New York City to Bergen County, New Jersey, US, as the world's busiest bridge, carrying 102 million vehicles annually; truss work between the roadway levels provided stiffness to the roadways and reduced movement of the upper level when the lower level was installed three decades after the upper level. The Tsing Ma Bridge and Kap Shui Mun Bridge in Hong Kong have six lanes on their upper decks, and on their lower decks there are two lanes and a pair of tracks for MTR metro trains. Some double-decked bridges only use one level for street traffic; the Washington Avenue Bridge in Minneapolis reserves its lower level for automobile and light rail traffic and its upper level for pedestrian and bicycle traffic (predominantly students at the University of Minnesota). Likewise, in Toronto, the Prince Edward Viaduct has five lanes of motor traffic, bicycle lanes, and sidewalks on its upper deck; and a pair of tracks for the Bloor-Danforth subway line on its lower deck. The western span of the San Francisco-Oakland Bay Bridge also has two levels.

Robert Stephenson's High Level Bridge across the River Tyne in Newcastle upon Tyne, completed in 1849, is an early example of a double-decked bridge. The upper level carries a railway, and the lower level is used for road traffic. Other examples include Britannia Bridge over the Menai Strait and Craigavon Bridge in Derry, Northern Ireland. The Oresund Bridge between Copenhagen and Malmö consists of a four-lane highway on the upper level and a pair of railway tracks at the lower level. Tower Bridge in London is different example of a double-decked bridge, with the central section consisting of a low-level bascule span and a high-level footbridge.

A viaduct is made up of multiple bridges connected into one longer structure. The longest and some of the highest bridges are viaducts, such as the Lake Pontchartrain Causeway and Millau Viaduct.

Multi-way bridge

A multi-way bridge has three or more separate spans which meet near the center of the bridge. Multi-way bridges with only three spans appear as a "T" or "Y" when viewed from above. Multi-way bridges are extremely rare. The Tridge, Margaret Bridge, and Zanesville Y-Bridge are examples.

Bridge types by use

A bridge can be categorized by what it is designed to carry, such as trains, pedestrian or road traffic ( road bridge), a pipeline or waterway for water transport or barge traffic. An aqueduct is a bridge that carries water, resembling a viaduct, which is a bridge that connects points of equal height. A road-rail bridge carries both road and rail traffic. Overway is a term for a bridge that separates incompatible intersecting traffic, especially road and rail. A bridge can carry overhead power lines as does the Storstrøm Bridge.

Some bridges accommodate other purposes, such as the tower of Nový Most Bridge in Bratislava, which features a restaurant, or a bridge-restaurant which is a bridge built to serve as a restaurant. Other suspension bridge towers carry transmission antennas.

Conservationists use wildlife overpasses to reduce habitat fragmentation and animal-vehicle collisions. The first animal bridges sprung up in France in the 1950s, and these types of bridges are now used worldwide to protect both large and small wildlife.

Bridge types by material

The materials used to build the structure are also used to categorize bridges. Until the end of the 18th century, bridges were made out of timber, stone and masonry. Modern bridges are currently built in concrete, steel, fiber reinforced polymers (FRP), stainless steel or combinations of those materials. Living bridges have been constructed of live plants such as Ficus elastica tree roots in India and wisteria vines in Japan.

Bridge type

Materials used

Cantilever

For small footbridges, the cantilevers may be simple beams; however, large cantilever bridges designed to handle road or rail traffic use trusses built from structural steel, or box girders built from prestressed concrete.

Suspension

The cables are usually made of steel cables galvanised with zinc, along with most of the bridge, but some bridges are still made with steel- reinforced concrete.

Arch

Stone, brick and other such materials that are strong in compression and somewhat so in shear.

Beam

Beam bridges can use pre-stressed concrete, an inexpensive building material, which is then embedded with rebar. The resulting bridge can resist both compression and tension forces.

Truss

The triangular pieces of truss bridges are manufactured from straight and steel bars, according to the truss bridge designs.

Military bridge

The Tank bridge transporter (TBT) has the same cross-country performance as a tank even when fully loaded. It can deploy, drop off and load bridges independently, but it cannot recover them.

Images for kids

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For other meanings of "bridge" see bridge (disambiguation) A bridge is a structure to cross an open space or gap. Bridges are mostly useful for crossing rivers, valleys, or roads by vehicles but people have also used bridges for a long time for walking. Bridges called highway overpasses are used to avoid traffic control.

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There are more than half a million bridges in the United States, and you rely on them every day to cross obstacles like streams, valleys, and railroad tracks. But do you know how they work? Or why some bridges are curved while others are straight? Engineers must consider many things -- like the distance to be spanned and the types of materials available -- before determining the size, shape, and overall look of a bridge.

Since ancient times, engineers have designed three major types of bridges to withstand all forces of nature.

Cantilever bridge:
Firth of Forth

consists of an assembly of triangles. Truss bridges are commonly made from a series of straight, steel bars. The Firth of Forth Bridge in Scotland is a cantilever bridge, a complex version of the truss bridge. Rigid arms extend from both sides of two piers. Diagonal steel tubes, projecting from the top and bottom of each pier, hold the arms in place. The arms that project toward the middle are only supported on one side, like really strong diving boards. These "diving boards," called cantilever arms, support a third, central span.

Check out the forces that act on truss bridges!

Ancient Roman aqueduct

Suspension bridge:
Golden Gate Bridge

Now that you've mastered the bridge basics, test your bridge-building skills in the Bridge Challenge!

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There are more than half a million bridges in the United States, and you rely on them every day to cross obstacles like streams, valleys, and railroad tracks. But do you know how they work? Or why some bridges are curved while others are straight? Engineers must consider many things -- like the dist…

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You got here just in time. This growing community needs four new bridges, and they're very picky about what they want!

Test your engineering skills and try to match the right bridge to the right location.

Click on one of these four regions to get started. By the way, you may use each bridge type only once -- that's a direct order from the mayor of Craggy Rock. Good luck!

Location 1: Multi-lane bridge for commuters and tourists
Location 2: Footbridge across a stream
Location 3: Highway bridge across a busy shipping port
Location 4: Railroad bridge in a national park

Want some help? Stop by Bridge Basics before you begin.

Having technical difficulties? Ask for help.

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You got here just in time. This growing community needs four new bridges, and they're very picky about what they want! Test your engineering skills and try to match the right bridge to the right location. Click on one of these four regions to get started.

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The beam bridge is the oldest, simplest, and most common type of bridge. A beam bridge is a horizontal, or level, structure. It has a support on each end. Other supports, called piers, may also hold up the bridge between the two ends. A log or a plank that crosses a stream is a beam bridge in its most basic form. Modern beam bridges are usually made of steel beams, called girders. Most highway bridges are beam bridges.

Like the beam bridge, the truss bridge has a support at each end. It may also rest on piers in between. But its structure gives it more strength than a simple beam bridge. A framework of metal or wood bars connects the two ends of the bridge. These bars fit together in triangular shapes. They often form a sort of tunnel through which the roadway passes.

The cantilever bridge is made up of structures called cantilevers. A cantilever is a beam that has a pier at only one end, like a diving board. A framework of many bars adds strength to the beam, as in a truss bridge. At least two of these beams stretch toward each other to form a cantilever bridge.

Like the beam bridge, the arch bridge is a very old design. An arched structure built beneath the bridge's roadway provides its support. Arch bridges often span rivers and valleys.

In the suspension type of bridge, the roadway hangs from strong wires called cables. The main cables hang between two or more towers. Smaller cables hang down from the curving main cables. The smaller cables hold up the roadway. Suspension bridges can span longer distances than any other type of modern bridge.

A cable-stayed bridge also uses cables to support the roadway. Its cables run directly between the towers and the roadway. They attach to the roadway in straight, diagonal lines.

Some types of bridges are movable. Some may open upward to allow tall ships to pass underneath. Others may turn sideways. Still others, called pontoons, float on water.

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The beam bridge is the oldest, simplest, and most common type of bridge. A beam bridge is a horizontal, or level, structure. It has a support on each end. Other supports, called piers, may also hold up the bridge between the two ends.

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Gain instant access to this beautifully designed Unit on Bridges, where kids will learn all about How Arch and Suspension Bridges Work, Bridges Around the World, and more. Access 3 different reading levels perfect for Grades 3-8. Written by subject experts, aligns with standards.

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What Makes Bridges So Strong? | Engineering for Kids | STEAM | SciShow Kids

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Arch Bridge

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Types of Bridges